There is continuing demand for manufacturing processes that may lessen the cost, time or steps in producing a part. More often than not, the benefits resultantly associated with improving the manufacturing process are necessitated in the first instance by customer requirements to develop and improve products to have superior dimensional, mechanical and/or performance properties. For instance a typical differential side gear may have any or all of the following performance requirements such as: the spline area requiring dimensional precision, high shear strength and brinnelling resistance; the hub and thrust faces requiring dimensional precision, surface finish and case compatibility; the gear geometry requiring dimensional precision, surface finish and optimised profile; and the tooth and core strength may require impact resistance, wear resistance, spalling resistance, and different surface and core metallurgies. Different non-compatible manufacturing processes, i.e. casting, steel forging or powder metal forging, obtain different performance requirements for the same part, advantageously or otherwise.
Referring to FIG. 1, in order to meet some of these performance requirements, a gear 10 is made by forging a powder metal 14 and then case carburizing the gear to achieve a nearly constant effective case depth 16. The case depth 16 for each gear tooth 12 is shown in the partial cross-sectional view of FIG. 1. However, a case carburized gear does not necessarily achieve the desired mechanical properties such as enhanced tooth wear and fatigue strength while providing beneficial performance characteristics in the body of the gear that could beneficially result if the part was made from composite materials. It would be advantageous to achieve an improved balance of these performance requirements in the final product, uncompromised by the manufacturing process thereby saving time, processing or cost.
A manufacturing process to improve the performance requirements of powder metal parts is disclosed in U.S. Pat. No. 5,903,815 titled “Composite Powdered Metal Component.” The process teaches a component made from two or more discreet powder metals that are compacted and then sintered. Also, U.S. Pat. No. 6,148,685 titled “Duplex Sprocket/Gear Construction and Method of Making Same,” discloses a method of making a gear made from two or more discrete powder metals that are then also compacted and sintered to obtain the final part. These patents only teach composite powdered metal parts obtained by sintering, in general. No known processes exist which address applying composite powder processes to improve performance features such as improved load bearing upon the tooth flank of a gear while providing impact resistance and bending fatigue resistance in the tooth root; providing a composite variable boundary layer; or making a substantially fully dense part by compaction forging.
Therefore, there is a need for a composite powder metal variable boundary part exhibiting improved impact resistance and bending fatigue resistance in the tooth root. It would also be advantageous to provide a composite variable boundary layer or a substantially fully dense part by compaction forging. There is also a need for a method of producing a variable boundary in a composite powder metal part.